To assess whether a combination of low-intensity, fixed dose Warfarin and aspirin is equally effective but safer than adjusted-dose Wafarin in AF pts predicted to have a high risk of stroke. A large fraction of patients with atrial fibrillation have relatively low rates of thromboembolism during aspirin therapy, and these can be identified by clinical criteria. Additional studies using transesophageal echocardiography may clarify pathogenetic links and further refine risk stratification. Selection of antithrombotic prophylaxis for atrial fibrillation patients should consider the widely different rates of the thromboembolism associated with individual patient features to provide optimum patient safety.